1.4 INTERMOLECULAR FORCES Flashcards
(18 cards)
What are intermolecular forces?
Refers to any type of bonding interaction between neighbouring molecules
What are intramolecular forces?
Refers to any type of bonding interaction within a molecule
What are van der Waals forces?
Refers to the attraction between instantaneous and induced dipoles on neighbouring molecules.
Explain how van der Waals forces of attraction arise between molecules
Temporary dipoles caused by random movement of electrons.
How are instantaneous and induced dipoles formed and explain their subsequent attraction.
1) Electrons in atoms/molecules are travelling at high speeds in orbitals
2) As two molecules approach each other, the electrons in one molecule repels the electrons in the other to one side
3) This forms a dipole. One side is slightly more -ve and the other slightly more +ve
4) A dipole in one molecule can induce a dipole in a neighbouring molecule
5) The molecules are then attracted to each other by their oppositely charged ends/partial charges.
How does Mr affect mpt/bpt?
- As Mr increase, the number of electrons also increase. this causes molecules to have larger dipoles therefore stronger van der Waals forces of attraction between neighbouring molecules.
- This means more energy is required to overcome the van der Waals forces hence the higher mpt/bpt required.
How does branching affect mpt/bpt?
- As branching increases, the contact SA between electrons on neighbouring molecules has been reduced therefore decreases the van der Waals forces of attraction.
- This means that there is less energy required to overcome the van der Waals forces hence the lower mpt/bpt required.
How does branching affect the viscosity of organic molecules.
As branching increases, the viscosity decreases. this is because branching causes decreased van der Waals forces of attraction.
How is viscosity affected by intermolecular forces?
The greater the (intermolecular force) between the molecules, the greater the viscosity.
What are Permanent dipole- Permanent dipole attractions?
The attraction between the ∆+ve end of a permanent dipole on one molecule and the ∆-ve end of a permanent dipole on a neighbouring molecule.
Explain in terms of intermolecular forces the difference between melting points between non-polar molecules and a polar molecule.
The bpt of polar molecules are higher as the combination of van der Waals forces of attraction and dipole-dipole attractions between neighbouring molecules are stronger than just the van der Waals forces of attraction between neighbouring molecules in non-polar molecules.
Explain Hydrogen bonding
- The attraction between a lone pair of electrons on a very electronegative atom (i.e. N,O, F) in one molecule and a hydrogen atom in a neighbouring molecule, in which the hydrogen atom is covalently bonded to a very electronegative atom (N,O,F).
- High bpts in molecules containing Hydrogen bonds indicates that Hydrogen bonds are significantly stronger than dipole-dipole attractions.
Why does ice have a lower density than water?
-Hydrogen bonds in ice are fixed and the water molecules are in a widely spaced lattice leading to a more open structure hence a lower density.
Explain the structure of ice
Each water molecule is hydrogen bonded to four other water molecule in a tetrahedral arrangement.
What is miscibility?
Describes liquids that mix in all proportions to form one layer
What is immiscibility?
Describes liquids that are unable to mix together and forms two distinct layers.
What is the “like dissolves like” principle
Liquids will mix if the intermolecular forces in each liquid are similar.
Miscibility of ionic compounds
Metal ions have a +ve charge and form ion-dipole attractions with the ∆-ve end of the dipoles of polar molecules in the liquid.
Non-metal ions either form ion-dipole attractions or hydrogen bonding depending on the anion with the ∆+ve end of the dipoles of the polar molecules in the liquid .
